Composition and Value of Guano. 



215 



apparent, the mineral phosphate will be first considered. Leav- 

 ing out of the question the great bed of Estremadura phosphorite, 

 which at present is not of sufficiently ready access to prove of any 

 practical importance to British agriculture, there are two chief 

 supplies of mineral phosphate of lime in England— the phos- 

 phoric nodules or " coprolites" of the " crag" beds, and the 

 somewhat similar deposits in the " green-sand" formation. 



The former has up to a late period stood alone in practical im- 

 portance, and it yet remains to be seen whether the phosphoric 

 strata of the chalk and green-sand will at any future time offer a 

 more available supply of phosphate of lime. We may shortly 

 consider both of these sources. 



The coprolites of Suffolk occur in rounded lumps and masses 

 in a shelly gravelly soil, and in many places very near the sur- 

 face. They are separated from the gravel by an easy process of 

 sifting. 



These coprolites are sold in London, with a profit to the col- 

 lectors, at about 30s. a ton. 



To grind them into a tolerably fine powder (and as they are 

 intensely hard, this operation requires very powerful machinery) 

 a further expense of from 15s. to 20s. a ton is incurred. It is 

 probable that the ground coprolites could not be sold at a much 

 lower rate than 31. a ton. But it is to be remembered that 

 the coprolites are not entirely composed of phosphate of lime ; 

 on the contrary, not much above one-half of them consists of this 

 substance. 



The average proportion of phosphate of lime in the coprolites 

 is 56 per cent. A ton of them contains therefore 1532 lbs., and 

 by an easy calculation it will be found that a ton of phosphate of 

 lime as supplied by ground coprolites will cost 5/. 9<y. Id., which 

 is at the rate of 100 lbs. for 4s. I0d., or rather more than hd. for 

 each pound of phosphate of lime. 



Turn we now for a minute to the other deposit of mineral phos- 

 phate of lime. 



At the junction of the green-sand with the gault clay are 

 conglomerated masses or nodules, of various sizes and forms, made 

 up of phosphate of lime and sand. These have been fully de- 

 scribed in the Journal of the Society, vol. ix., part i. 



I am not aware that these remains have been collected in any 

 quantity for sale, and at present any conjectures as to their cost 

 must necessarily be open to much error. Mr. Paine, who has 

 dug 50 or 60 tons for his own use, has found them to cost him 

 in labour, when delivered at his farm by his own men and horses, 

 about 15s. a ton. 



These are not nearly so hard as those of the crag, and would 

 probably not cost so much to grind. Let us suppose that they 



